1. Field of the Invention
The present invention relates to magnetic disk drives, and methods for forming a disk stack assembly with concentrically-aligned disks.
2. Description of Background Information
Magnetic disk drives store data for use by all types of computer systems. A magnetic disk drive typically has a case, enclosing one or more spinning disks (also known as “platters”), arranged one on top of another in a stack. An actuator arm of the hard disk drive responds to commands the drive receives from a host computer, and the actuator arm moves to a proper location on the disk. When the actuator arm arrives at the proper location, a read/write head attached to the actuator arm either reads data from or writes data to the location.
A typical magnetic disk drive comprises a series of aluminum alloy disks. Although some magnetic disk drives use only a single disk, most use a plurality of disks. The disks have a central opening. The disks comprise a substrate material that forms the bulk of the disk, and a magnetic media coating which holds the magnetic pattern that represents the data.
The disks may be mounted on a common central spindle to form a disk stack assembly that spins as one piece. The disk stack assembly may comprise a spacer ring between adjacent disks on the spindle. The spacer ring provides a longitudinal separation between adjacent disks.
It is important to center the disks on the spindle so that they do not cause an imbalance and excessive vibration when rotated at high speeds. An out-of-balance condition in the rotating disk stack assembly can cause erratic speed variations in the movement of the disks in relation to the read/write heads, which may produce read/write errors. Vibrations of the rotating disks can also contribute to head crashes, harming both the disk and read/write head.
There are known processes for centering and balancing disks when securing them to the spindle of a disk stack assembly. In one such stack assembly build process, the inner diameter (ID) of a central opening of each disk is specified to have some value (e.g., two mils) greater than that of the outer diameter (OD) of the spindle. This ID-OD difference between the ID of the disk and the OD of the spindle is chosen to prevent the disk from binding to the spindle even when there is a slight lack of perpendicularity between the disk and the axis of the spindle during assembly. An averaging procedure is employed to cancel offsets of disks against each other. In this process, these parts may be misaligned by up to approximately one mil. For some high-performance products, this misalignment is reduced by employing a dynamic balancing process.